Selecting optional
axle ratios when buying a new truck not as bad on fuel
economy as some imagine
Hang
around a car lot for any amount of time and you will hear
buyers automatically ask about a potential vehicles
fuel economy. But after the purchase, the subject seldom
arises in conversation as people come to realize that in
real world driving, fuel economy is based on how hard a
pickup or SUV has to work to pull the load.

That is where
axle ratios come into play. This factory option is all
too often passed over becaue new truck buyers mistakingly
think that the "lower" ratio automatically mean
poorer fuel economy.

So the majority
of pickup and SUV owners buy the vehicle with the
standard or base axle ratio, which is
typically 3.31:1, 3.42:1 or 3.55:1. Such thinking is more
likely to lead to dissatisfaction with the trucks
overall performance than its fuel economy.

Manufacturers offer
optional axle ratios for one reason: to improve the
vehicles pulling performance. The added benfit,
however, is fuel economy doesn't take much of a hit--if
any at all.

According to Roger Clark,
senior manager for General Motors Integration and Fuel
Economy Learning Vehicles Program (FELVP), which handles
EPA testing of all GM trucks and SUVs, "fuel economy
may not change at all by going to an optional axle ratio.

The
typical combined fuel economy impact, based on EPA
lab test conditions, is about .4mpg to .6mpg between the
base gear ratio and the lowest (4.10) offered. That
change is linear. Equipping a truck or SUV with a 3.73
gear ratio, for instance, would affect combined fuel
economy by less than a quarter-mile-per-gallon.

Those fractions of a
mile-per-gallon will never be noticed by an owner.
Further more, Clark says that in the real world, choosing
a lower gear ratio may not even show up in city
driving fuel economy. Its the steady-state,
long-distance freeway trips where the lower axle ratios
have the most affect on fuel economy.

The
reason we [GM] offers the 3.42 and 3.55 ratios in our
pickups and SUVs is those ratios offer the best fuel
economy with a four-speed automatic transmission. If you
want a truck that responds best to a heavy load or towing
a trailer, then 3.73, 3.92, or 4.10 ratios will provide
the best wheel torque at the lower engine speeds.

A lower
(numerically higher) gear ratio provides more low-speed
wheel torque, which makes it easier to get the vehicle
moving when pulling a trailer or carrying a load of
passengers and cargo. Lower gears also improve
acceleration up to about 60mph. (Passing performance and
speeds above 60mph is a matter of horsepower, not axle
ratio.)

As for general
fuel economy, Clark says, Sure, most of the axle
ratio changes are going to have a cumulative effect in
fuel economy over time, but its going to be very
hard to notice any difference in fuel economy where the
vehicle is being driving in a mix of city and highway
conditions.

The reason is
aerodynamic drag from the trucks frontal area, not
the gear ratio, is the major contributing factor in fuel
economy.

According to
the EPA, during the highway test cycle where the average
speed is 48 mph, 54% of the engines power is used
to overcome aerodynamic drag. Drive faster and the engine
has to work even harder to push through the air and
consumes more fuel in doing so.

The best fuel economy for
the typical truck or SUV is cruising right around 40mph.
The EPA test cycle for the highway fuel economy number
averages 48mph with a top speed of 60mph, explains
Clark.

A good
example of how drag affects fuel economy is a truck that
has a 21mpg highway EPA number. Drive at a 10-percent
higher average speed (53mph) and drag causes fuel economy
to fall about 1.5mpg. Average 60mph and mileage will drop
another 1.5mpg. Run just above70mph and now fuel economy
is less than 14mpg instead of 21mpg.

So, if fuel
economy is of a real concern, watch your speed. If
performance is your goal, watch the axle ratio you
buy.---Bruce W. Smith